1. Field of the Invention
The present invention relates in general to a method and circuit board structure for leveling solder balls in BGA (ball grid array) semiconductor packages with solder ball input and output terminals. More particularly, the present invention relates to an improved method and circuit board structure for improving coplanarity of a BGA package's solder ball array caused by thermal bending of the circuit board generated during a high temperature processing step of a BGA semiconductor package producing process.
2. Description of the Prior Art
In recent years, several types of BGA (ball grid array) semiconductor packages with solder ball input and output terminals, for example, cavity BGA packages, die down BGA packages and tape BGA packages have been proposed and used as new semiconductor packages. In the typical BGA semiconductor packages, PCBs (printed circuit boards) and polyimide tapes are generally used as the circuit boards of the packages. However, the BGA package's parts such as the above circuit boards are apt to be applied with and affected by the thermal stress during high temperature steps (performed under the condition of a temperature not lower than 150.degree. C.) of a semiconductor package producing process. FIGS. 1A to 1D show examples of bad coplanarity of the solder ball array as a result of thermal bending of a typical BGA semiconductor package. As shown in the drawings, the typical BGA package is bent when the package is applied with the thermal stress during a high temperature step of the package producing process. This is due to different thermal expansion coefficients of the PCB 1 and plastic body 2. As the typical BGA packages have been generally molded by a one-side molding method in which only one side of the package is molded, the thermal bending of the package becomes worse. The array of solder balls 3 welded to the PCB 1 is bent by the thermal bending of the PCB 1. The solder ball array thus has bad coplanarity. When the coplanarity of the solder ball array is lower than a predetermined level, a part of solder balls 3 fails to come into contact with a mother board not shown when the package is mounted to the mother board. For example, when the BGA package is thermally bent to cause the top center of the package to be concaved as shown in FIG. 2A, the solder balls 3 welded to both end portions of the PCB 1 fail to come into contact with the mother board when mounting the package to the mother board. On the other hand, when the BGA package is thermally bent to cause the top center of the package to be convexed as shown in FIG. 2B, the solder balls 3 welded to the center portion of the PCB 1 fail to come into contact with the mother board. When a part of the solder balls 3 is not brought into contact with the mother board due to the thermal bending as described above, the package fails to perform its operational function.
In an effort to prevent the above thermal bending of the plastic body and PCB of the BGA package, the configuration of the plastic body 2 is changed from the typical square or rectangular configuration into another configuration capable of effectively resisting the thermal stress. For example, the corners of the plastic body 2 may be smoothly rounded. In addition, the slope angle .THETA. of the sides of the plastic body 2 increases from 7.degree.-12.degree. degree to 25.degree.-35.degree. degree as shown in FIGS. 3 and 4, thus reducing the stress of the body's sides and minimizing the thermal bending of the PCB 1 and body 2.
However, the above method for preventing the thermal bending of the body and PCB is not for improving the ratio of contact of the solder balls, but for protecting the semiconductor chip and preventing cracks in the package's parts. In this regard, the above method somewhat prevents the thermal bending of the plastic body but fails to prevent the thermal bending of the PCB when the package is thermally bent as shown in FIGS. 1C, 1D and 2B.